1. Field of the Invention
The present invention relates to a multicolor image forming apparatus such as a copier or a printer adapted to form a multicolor image from toner images produced by a plurality of unicolor imaging units installed removably in the apparatus body.
2. Description of the Related Art
Regarding a multicolor image forming apparatus, there is recently proposed a type where a plurality of unicolor imaging units with photosensitive drums, developing devices and so forth incorporated integrally therein are installed removably in the apparatus body, and a full-color image is formed by successively transferring toner images, which are produced individually by such imaging units, to a recording paper transported by means of a conveyor belt or the like.
In such a multicolor image forming apparatus, it is essential to prevent a positional color deviation caused due to the process that forms a multicolor image by successively transferring individual toner images obtained from the plural imaging units and then superposing the toner images mutually on the recording paper. A variety of considerations need to be given for prevention of such color deviation, and particularly in the multicolor image forming apparatus of the type mentioned, the prime requisite is that the transfer positions in the entire imaging units are mutually constant. More specifically, the positional relationship among photosensitive drums in the imaging units and also among the individual drums should be retained constant in the apparatus.
However, the multicolor image forming apparatus mentioned has such a construction that photosensitive drums are incorporated removably in the unicolor imaging units respectively, and the unicolor imaging units themselves are installed also removably in the apparatus body while being supported by means of slide rails. Therefore, particularly when the imaging units are installed in the apparatus, the photosensitive drums may fail to be set exactly at predetermined positions in the apparatus. Accordingly it is necessary to adopt some adequate measures for precisely placing each drum in a fixed positional relationship to the apparatus body.
Besides the above, it is requisite in such unicolor imaging unit that a photosensitive drum and a roll of a developing device need to be spaced apart from each other with a proper gap required for development.
Practically, however, the gap for development is varied between the imaging units as well as in any single unit itself because of the construction where the photosensitive drums are anchored removably to unit frames in the stage of installation of the imaging units in the apparatus body, and also the imaging units are installed removably in the apparatus body while being supported by means of slide rails, and further due to the tolerances of individual component parts existing between the photosensitive drum and the developing device (roll). And such variation of the gap raises a problem that desired development fails to be performed properly to consequently cause some uneven image density in the individual imaging units for thereamong. The gap variation thus induced tends to worsen with an increase of the frequency of repeated installation and removal of the imaging units or the photosensitive drums.
Therefore, it has been necessary in the conventional apparatus to maintain the required dimensional precision of individual component parts and also the required high accuracy in assembling and attaching such component parts with utmost strictness and advanced skill for the purpose of retaining the gap constant between the photosensitive drum and the developing roll. In this case, however, there exist some problems including an increase in the production cost of the component parts and more difficulty in handling the same.
As a means for maintaining the gap constant between the photosensitive drum and the developing roll, there is known a technique of providing, at both ends of the roll, tracking members which project therefrom at a height equal dimensionally to the gap, and keeping such tracking members in contact with the surface of the photosensitive drum to retain the desired gap. However, in any imaging unit employing none of such means, maintenance of the desired gap for development is rendered further essential.